This relates generally to imaging devices, and more particularly, to imaging devices having analog-to-digital converters (ADCs).
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns. Circuitry is commonly coupled to each pixel column for reading out analog image signals from the image pixels. Analog-to-digital converters are used to convert the analog image signals to corresponding digital image data.
The ADCs are often dual-slope ADCs having an integrator and a comparator that provide an output to a counter. The counter is controlled using rising and/or falling edges of a clock signal. During a first integration period, from a common-mode level, the integrator integrates an analog image signal for a period of time. This period of time will determine the bit resolution of the conversion. A second integration period starts from a voltage level at the end of the first integration period. During the second integration period, a reference signal, instead of the analog image signal as in the first integration period, is applied to the integrator input. The second integration period ends when the integrator output crosses the common-mode level of the comparator. The durations of both the first and second integration periods are measured. Digital data corresponding to the analog image signal is obtained using a ratio of the durations of the first and second integration periods. A counter outputs the digital data as the ADC's digital outputs.
In conventional dual-slope ADCs, there is a trade-off between conversion time and resolution in their implementation. Consequently, it can be difficult to perform both high-speed conversion at a low bit resolution and high-speed conversion at high bit resolution with satisfactory noise performance. In addition, clocking the ADCs using rising and/or falling edges of a clock signal imposes an inherent limit to the speed with which the data is converted.
It would therefore be desirable to be able to provide imaging devices with improved analog-to-digital converter circuitry.